1. Field of the Invention
This invention relates to an electrophotographic apparatus in which a print time required for a first print sheet is shortened.
2. Description of the Related Art
One type of electrophotographic apparatus has been conventionally a laser printer as shown in FIG. 8. This laser printer mainly comprises an image forming device, a fixing device 24 and a sheet feeding device.
The image forming device includes a photosensitive drum 18, a charger 15 for charging the photosensitive drum 18, a scanner unit 19 for carrying out an exposure operation to form an image on the photosensitive drum 18 in accordance with a print data, a developing device 20 for developing an electrostatic latent image formed through the exposure process to form a toner image on the photosensitive drum 18, a transfer unit 21 for transferring the toner image developed on the photosensitive drum 18 onto a sheet 11, and a discharger 22 for discharging the sheet 11 to exfoliate the sheet 11 from the photosensitive drum 18 after the transfer process.
The fixing device 24 includes a heat roller 25 for fixing the toner image on the sheet 11, a heater 26, which is secured to the heat roller 25 and serves to heat the heat roller 25, and a temperature sensor 31, which is secured to the frame of the fixing device so as to be in contact with the heat roller 25 under pressure and serves to detect the temperature of the heat roller 25. The fixing device 24 is further provided with a support roller 32 that is paired with the heat roller 25. A fan motor 30 is disposed in the vicinity of the fixing device 24.
The sheet feeding device serves to connect the elements as described above, and feed the sheet 11 to each element. It includes an automatic sheet supply device 10, a sheet guide member comprising a sheet guide 12, etc., and a roller member comprising a register roller 14, etc.
The print operation of the laser printer thus constructed will be described along the path of the print sheet with reference to FIG. 8.
Upon reception of a print instruction, the heater 26 is first actuated, and the temperature sensor 31 detects the temperature of the heat roller 25. After the temperature of the heat roller 25 exceeds a predetermined fixing temperature, which is required to sufficiently fix the toner image onto the sheet 11, the scanner unit 19 is actuated. After a while, the rotational speed of a scanner motor built in the scanner unit 19 reaches a constant high rotational speed, the photosensitive drum 18 is rotated at the time when the exposure process is allowed to be carried out, and a high voltage is applied to the charger 15 to charge the surface of the photosensitive drum 18. After the photosensitive drum 18 makes one revolution and the surface of the photosensitive drum 18 is homogeneously charged by charger 15, the feeding of the sheet 11 is started.
The sheet 11 is supplied from the automatic sheet supply device 10, which is provided at a lower portion inside of the main body 1 of the laser printer and fed to the image forming device while being guided by the sheet guide 12. When the sheet is detected by a sheet detection sensor 13, a pair of resist rollers 14, which are provided downstream of the sheet detection sensor 13, start their rotation after a predetermined time lapse on the basis of a sheet detection signal from the sheet detection sensor 13. The sheet 11 is contacted with the resist rollers 14 to correct its oblique orientation and then fed out from the resist rollers 14 to the downstream side in synchronism with the rotation of the resist rollers 14. Thereafter, the sheet 11 is guided by a sheet guide 16 provided downstream of the resist rollers 14 and fed to the image forming device by a carry roller 17 provided downstream of the sheet guide 16. The sheet 11 is guided in close contact with the photosensitive drum 18.
In the image forming device, print data is formed as an electrostatic latent image on the photosensitive drum 18 by scanning the photosensitive drum 20 with a laser beam from the scanner unit 19. Further, in the developing device 20, toner supplied from the developing device 20 is absorbed onto the electrostatic latent image by the electrostatic charge of the electrostatic latent image to thereby develop the electrostatic latent image.
By charging the sheet 11 in the transfer unit 21, a toner image that is visualized in the developing device 20 is transferred onto the sheet 11. After the transfer process, in the discharger 22, the charged sheet 11 is and exfoliated from the photosensitive drum 18 to which the sheet 11 is closely contacted. Thereafter, the exfoliated sheet 11 is guided by a feeding guide unit 23 disposed downstream of the image forming device and fed to the fixing device 24 provided further downstream of the image forming device.
The sheet 11, which is fed from the feeding guide unit 23, is sandwiched between the heat roller 25 maintained at a predetermined temperature and the support roller 32, and it is fed toward a pair of feed-out rollers 33 provided downstream of the heat roller 25 and the support roller 32. At this time, the toner image on the sheet 11 is fixed onto the sheet 11 by the heat of the heat roller 25. Subsequently, the sheet 11 is fed from the fixing device 24 to a sheet discharge unit 27.
The sheet 11 is guided to the sheet discharge unit 27 and fed onto a sheet discharge tray 29 through rotation of a pair of sheet discharge rollers 28. After the above operations are carried out, the printing operation is terminated.
During printing, the temperature of the heat roller 25 is detected by the temperature sensor 31. If the detected temperature is below a predetermined temperature, the heater 26 is turned on. On the other hand, if the detected temperature is above the predetermined temperature, the heater 26 is turned out. That is, the temperature of the heat roller 25 is controlled to be constant. However, when the heater 26 is turned on, the heat of the heater 26 causes the temperature inside of the main body 1 of the laser printer to be also increased, and thus, the scanner unit 19 and other elements are also heated. Therefore, the output power of the laser beam is fluctuated, and the printing operation suffers adversely. In order to avoid such an adverse operation, the laser printer is provided with a fan motor 30, and the warmed air in the main body 1 of the laser printer is discharged to the outside of the main body 1 of the laser printer by rotating the fan motor 30, thereby cooling the inside of the main body 1 of the laser printer.
In the printing process as described above, it takes a significant amount of time to complete a printing operation for a first sheet (hereinafter referred to as "first print time"). That is, a first time of several seconds is required until the temperature of the heat roller 25 reaches the predetermined value, a second time of several seconds is further required until the scanner unit 19 is allowed to carry out the exposure operation, a third time of several seconds is further required from the time when the high voltage is applied to the photosensitive drum 18 until the time when the photosensitive drum 18 can photosensitize light, and a fourth time of several seconds is further required until the sheet 11 is fed to a predetermined position where the toner image is transferred onto the sheet 11. Accordingly, as much as 22-24 seconds (the sum of the first, second, third and fourth times) are required to prepare for the printing operation of the first sheet (hereinafter referred as "first printing operation").